The present invention relates generally to methods of forming niobium monoxide and, more particularly, to methods of depositing niobium monoxide suitable for use as MOSFET gates.
As the gate length of silicon CMOS devices are scaled below 100 nm, new high-k materials are expected to replace silicon dioxide as the gate insulating material, and new gate materials, including metal gates, are expected to replace polycrystalline silicon. These new gate materials are expected to solve the polysilicon gate depletion problem. The new gate materials may also enable threshold voltage adjustment without the need to alter the channel doping.
The new gate materials should have an appropriate work function. For CMOS devices, the gate materials should have a work function of approximately 4.2 eV for NMOS gates. The gate materials should have a work function of approximately 5.0 eV for the PMOS gates. It is possible to use different gate materials for the two different kinds of gates, NMOS and PMOS, in the CMOS process.
The new gate materials should be stable. NMOS metals are often highly reactive and normally unstable in contact with the gate dielectric. PMOS metals are more stable but more difficult to process. Even if chemically stable materials are identified, they should also be mechanically stable as well. The new gate materials should not exhibit poor adhesion. The new gate materials should not diffuse into the channel.
It would be desirable to have new gate materials that could be integrated into existing IC processes, such as having deposition and etching process that can be incorporated into existing processes.
The available choices for new gate materials include elemental metals, binary alloys, ternary alloys or even more complex materials. There are many materials that may qualify as PMOS gate candidates. However, qualified NMOS gate candidates, having a low work function, are very limited. One of the challenges for NMOS gate candidates is having a low enough work function, for example around 4.2 eV along with good stability in contact with the gate dielectric.
Binary alloys of RuTa or MoN have been explored. By altering the composition of these binary metal alloys it is possible to control the work function of the resulting material. However, when the work function of these alloys is targeted to values suitable for NMOS gates, work function below 4.3 eV, the thermal stability tends to deteriorate.